As the weather gets warmer, the risk of developing an infection at the site of a surgical wound increases, a new study reports.

Surgical site infections are a leading cause of increased hospital readmissions, longer length of hospital stay, greater health care costs and mortality.

Researchers used databases of hospital discharges across the United States to identify every adult hospitalization with a surgical site infection from 1998 to 2011, and tracked local temperature with data from the National Oceanic and Atmospheric Administration. The study, in Infection Control & Hospital Epidemiology, included more than 55 million hospitalizations in more than 2,500 hospitals.

Compared with January, the risk of being admitted to a hospital for a surgical site infection rose steadily from 9 percent higher in February to 21 percent higher in August. Then the risk declined in each month through December.

After controlling for many other variables, the researchers found that for every five-degree Fahrenheit increase in average monthly temperature, the risk of hospital admission for a surgical site infection increased by 2.1 percent.

Why this happens is not clear.

“There are other skin and soft tissue infections that are seasonal,” said the senior author, Dr. Philip M. Polgreen, an associate professor of medicine at the University of Iowa, “and there have been reports that the bacterial colonization of the skin may change from season to season. But we need more investigation to help us understand the mechanisms.”